The effect of aging on cognitive ability varies from one individual to the next. Recent studies suggest that one of the factors influencing individual differences in cognitive aging is the use of hormone replacement therapy (HRT). For example, recent findings from the Baltimore Longitudinal Study on Aging suggest that the use of estrogen replacement therapy (ERT) protects against age-associated declines in memory and against the development of Alzheimer's disease. One limitation of such studies is that the women who participated in them elected on their own to receive ERT. Research suggests that such women tend to be better educated and to receive better health care than do women who elected not to receive ERT. The bias is termed the "healthy user effect." To address this bias, we are recruiting postmenopausal women age 65 and older who are not currently receiving HRT, and we are investigating their cognitive functioning at two points, after 3 months of receiving ERT and after 3 months of receiving placebo. We are conducting a parallel study on men, age 65 and older. Men experience a gradual loss of testosterone, about 1% a year after age 40. Little is known about how testosterone replacement therapy affects cognition in older men, though there is some suggestion that the hormone may enhance spatial abilities. The aim of the study is to determine whether estrogen and testosterone enhance cognition and mood in healthy older adults. We predict that women will show improved verbal abilities and memory while receiving ERT, but no enhancement, or possibly a decrement, in spatial abilities. We predict that men will show improved visuospatial abilities with no enhancement in verbal abilities while receiving testosterone.